Drill bits are a very common tool used in the construction and maintenance industries. There are a wide variety of drill bits that have been developed. Spade type drill bits, which have a spade-like blade or cutter, are generally used in association with small portable electric or cordless drills but they may also be used in association with a dill press.
Spade drill bits are generally inexpensive as compared to auger or twist type drill bits. One of the reasons that spade drill bits are affordable is their simple design and manufacture. A spade drill bit is made by flattening a portion of a cylindrical bar by forging and the formation of cutting edges on the flattened section by grinding. The remainder of the unflattened bar is used to engage the drill by way of a chuck. In addition the spade drill bit may be modified through secondary forging steps thereby modifying the flattened section to include performance improving impressions, for example folds, grooves and bends.
Other attributes of spade drills over auger or twist type drills include convenience of storage and ease of resharpening the bits in the field.
In general, spade type drills have a shank region of cylindrical cross section. The end of the shank section may have either ground or forged flats to assist engagement of the drill in the chuck of the portable electric or cordless drill or drill press. The flattened end of the spade drill has two parallel opposing planar face regions, each having a leading face portion and a trailing face portion and an axially extending centre spur that points away from the shank end of the spade. The centre spur extends between the opposing planar face regions of the spade bit. Each opposing face of the spade is bounded by leading and trailing longitudinal edges, by leading and trailing shoulder edges and by leading and trailing shank edges.
The longitudinal sides between the longitudinal edges are slightly tapered toward each other toward the shank end of the bit, further, the longitudinal sides occur at an acute angle from the face region of the spade drill along the leading longitudinal edge.
The shoulder sides are generally radially located between the centre spur and the longitudinal sides and are sloped so as to occur at an acute angle from the face region of the spade drill along the leading shoulder edge.
As the leading and trailing shank edges of the spade are removed from all cutting activity, the side located between these edges is left unfinished beyond the as forged condition.
The centre spur has centre spur faces that are bounded by the face region of the spade drill and by leading and trailing centre spur edges, between the leading and trailing edges are the centre spur sides, the sides occurring at an acute angle from the centre spur faces along the centre spur leading edges.
The centre spur sides, longitudinal sides and shoulder sides occur at acute angles from their respective faces to provide relief for the centre spur leading edge, longitudinal leading edge and shoulder leading edge respectively during operation of the drill.
Additionally, side spurs are often provided. These spurs generally are extensions of the longitudinal sides extending beyond the shoulder sides and have leading and trailing faces that are continuations of the leading face portion and trailing face portions respectively of the face region of the spade. Further the inward side of the side spur Is a non-coplanar extension of the shoulder side of the spade drill.
In operation, with the spade drill bit installed in an electric or cordless drill or drill press the centre spur is the first part of the bit to engage the work whereupon the centre spur leading edges cut out a conical impression in the work initiating cutting and providing stability for the spade drill. Further advancement of the drill allows the side spurs, if present, to cut a circular "v" shaped groove in the work whereupon further advancement causes the shoulder side leading edges to engage the work and remove material between the centre spur and side spur. This action continues until the centre spur exits out the other side of the workpiece and the side spurs cut a circular exit hole. During cutting action where the longitudinal sides are engaged with the workpiece and particularly upon break through of the centre spur and side spurs from the workpiece, the longitudinal sides provide stability of the bit in the formed bore.
A review of the prior art reveals that considerable effort has been taken to provide increased cutting efficiency of the spade drill at all of the cutting edges.
For example U.S. Pat. No. 2,782,824 issued to Robinson on Feb. 26, 1957, shows a groove in the centre spur face along a side of the centre spur leading edge. However, the inside edge of the centre spur flute is generally parallel to the centre spur leading edge and there is not an increase in volume of the flute in the longitudinal direction. Alternatively, U.S. Pat. No. 3,997,279 issued to Porter on Dec. 14, 1976 shows a full centre spur flute that has concave sides proximate to the centre spur leading edge and the centre spur trailing edge. The concave sides extend from the tip to the face of the spade drill bit. However, there is a considerably reduced amount of material in this centre spur which leads to an increased likelihood of failure.
An alternate prior art spade drill bit disclosed in U.S. Pat. No. 4,682,917 issued to Williams on Jul. 28, 1987, shows a groove in the face of the spade along a side of the shoulder leading edge with side spurs extending in the direction of the centre spur. In addition, the leading face of the side spur is sloping in the direction of rotation.
Despite these and many other improvements to spade drills there still remain deficiencies and it is the ambition of this invention to overcome these. In particular it would be advantageous to have a spade drill bit that has good cutting characteristics, that can be easily sharpened by the end user and is relatively easy to manufacture.